add router to 404 page back to nearest component version

src/partials/body-404.hbs

@@ -3,12 +3,51 @@
 <article class="doc">
 <h1 class="page">{{{or page.title 'Page Not Found'}}}</h1>
 <div class="paragraph">
-<p>The page you're looking for does not exist. It may have been moved.</p>
+<p>The page you&#8217;re looking for does not exist. It may have been moved. You can{{#with site.homeUrl}} return to the <a id="return-link" href="{{{this}}}">start page</a>, or{{/with}} follow one of the links in the navigation above.</p>
 </div>
 <div class="paragraph">
-<p>If you arrived on this page by clicking on a link, please notify the owner of the site that the link is broken.
+<p>If you arrived on this page by clicking a link on another site, please notify the owner of that site that the link is broken.
 If you typed the URL of this page manually, please double check that you entered the address correctly.</p>
 </div>
 </article>
+<script>
+;(function (pathname, returnLink) {
+  if (!returnLink) return
+  var startPages = [
+    {{#each site.components}}
+    { name: "{{{./name}}}", title: "{{{./title}}}", url: "{{{@root.site.path}}}{{{./url}}}", prefix: "{{{@root.site.path}}}/{{{./name}}}" },
+    {{#each ./versions}}
+    {{#unless (eq ./version 'master')}}
+    { name: "{{{./name}}}", title: "{{{./title}}}", version: "{{{./version}}}", url: "{{{@root.site.path}}}{{{./url}}}", prefix: "{{{@root.site.path}}}/{{{./name}}}/{{{./version}}}" },
+    {{/unless}}
+    {{/each}}
+    {{/each}}
+  ].reduce(function (accum, it) {
+    var prefix = it.prefix
+    delete it.prefix
+    if (('version' in it) && !it.url.startsWith(prefix + '/')) {
+      var prefixSegments = prefix.split('/')
+      var versionSegmentIdx = prefixSegments.findIndex((segment) => segment === it.version)
+      prefixSegments.splice(versionSegmentIdx, 1, it.url.split('/')[versionSegmentIdx])
+      prefix = prefixSegments.join('/')
+    }
+    accum[prefix] = it
+    return accum
+  }, {})
+  var returnTo
+  var segments = pathname.split('/').slice(0, -1)
+  while (segments.length > 1) {
+    if ((returnTo = startPages[segments.join('/')])) break
+    segments.pop()
+  }
+  if (returnTo) {
+    returnLink.href = returnTo.url
+    returnLink.innerText =
+      'start page for the ' + returnTo.title + ' ' +
+      (returnTo.version ? returnTo.version + ' ' : '') +
+      'documentation'
+  }
+})(window.location.pathname, document.getElementById('return-link'))
+</script>
 </main>
 </div>